1. Field of the Invention
This invention relates generally to the area of solder fluxes such as those used in the microelectronics area, for example, in flip chip joining, sometimes referred to as C4 (controlled collapse chip connection) joining.
A common task in the manufacture of microelectronic components involves the manufacture of single chip or multi-chip modules having solder bumps or input/output pins, which are inserted into a substrate. The solder bumps or input/output pins provide the needed electrical connections to the integrated circuit chip or chips, which are subsequently connected to the substrate or carrier. In other presently known manufacturing processes, a chip is soldered directly to a printed circuit board. With either process, solder flux compositions have typically been applied in order to connect the component to the selected substrate, for instance, the printed circuit board. Flux compositions have been employed to remove oxides from the pins and to prevent the pins from oxidizing when subjected to elevated temperatures for soldering, thereby serving to maintain the electrical conductivity of the pins. Once the solder is applied, any flux compositions or residue remaining on the surfaces of the chip and/or substrates must be removed to provide as clean a substrate as possible. In the past, this has meant that an extra step of flux removal was necessary in the manufacturing process.
The soldering operation, in general, and flux removal, in particular, is increasingly difficult when applied to microelectronics. The pieces to be joined are extremely small, making cleaning, tinning, postcleaning and inspection difficult. In some cases, to avoid over heating, only the lead portion of the parts to be joined can be heated during the soldering operation. Cleaning and postcleaning are difficult due to the small size of the component, their large numbers, and the potential damage to the electronics by the cleaning solutions used, if any. Another problem source results from the fact that many of the known soldering fluxes are corrosive. In the environment of microelectronics, corrosion from any residual flux can ruin an extremely costly device.
Organic fluxes are typically based on water-insoluble rosin or water-soluble organic acid. Activated rosin fluxes are used in soldering electrical connections on printed circuit boards. Wave soldering is used for mass production circuit board soldering as by applying the flux, preheating the board, applying the solder, cooling the board and cleaning it to remove flux residue.
The flux residue usually comprises ionic (e.g., acidic or basic) substances, and is corrosive, or can hydrolyze to corrosive constituents in the presence of moisture. This can lead to short circuits, noise generation, etc., in the use of the circuit board product. These adverse results are effectively avoided by subjecting the soldered board to a cleaning step to remove the ionic substances.
Many of the organic water-soluble fluxes presently available contain corrosive materials such as halides. A flux composition that contains free halogen can result in conversion to hydroacids and corresponding halide ions by hydrolysis at the soldering temperature. Hydroacids can further react with organic materials present in the flux to free halide ions. Accordingly, if the flux residue is not entirely removed, it will lead to corrosion of the parts soldered.
Because of these problems, so-called xe2x80x9cnon-activatedxe2x80x9d rosin fluxes have been used in the past in the microelectronic environment. This has not generally provided an acceptable solution, however, since the pure rosin alone is limited in oxide removal capability and can require rework to produce an acceptable product.
To improve the pure rosin flux oxide removal capability, a number of xe2x80x9cactivatedxe2x80x9d or xe2x80x9cmildly activatedxe2x80x9d rosin fluxes have been developed. These products have several shortcomings, including the necessity of a cleaning step to ensure the removal of corrosive agents left behind after the soldering operation. For instance, it was often necessary to employ a hot water rinse in combination with neutralization or a mild hydrochloric acid solution in combination with a hot water rinse and neutralization or to use specialized water-based detergents. These cleaning steps caused difficulties during the assembly of chips to integrated circuit board where the low stand-off height of the chip to the substrate made it extremely difficult to clean underneath the chip with an aqueous or non-solvent process.
Other of the commercially available low residue fluxes have proved to be too thin, running out from under the chip during the manufacturing operation and failing to hold it in place. In other approaches a soldering flux is used that thermally dissipates after solder reflow so that no visible residue is left on the printed circuit substrate that would be visible with conventional inspection techniques such a light microscopy, or visual inspection at low powers of magnification.
Flip chip joining is the most viable interconnect scheme for high pin count single chip packages. The conventional process for performing such joining, practiced for many years by IBM and popularly known as xe2x80x9cC4,xe2x80x9d involves the use of a rosin-based flux, reflow in a hydrogen environment at approximately 350xc2x0 C., and cleaning of the flux residue using an aggressive solvent, such as hot xylene. In order to facilitate the adoption of C4 technology for cost-sensitive high pin count ASIC products, simplification of the overall process complexity and cost is considered to be critical. Notably, the facilitization of safe hydrogen environments, solvent cleaning equipment and the handling of aggressive chemicals is quite atypical of most package assembly environments and such factors present expensive entry barriers to the technology.
The formation of a flux residue during C4 joining is predominantly attributable to the presence of rosin in most commercial fluxes. The rosin, which is a distillation product of certain natural tree saps, is composed chiefly of abietic acid, which is known to decompose and transform to other more inert forms in the vicinity of 300xc2x0 C., which is below the typical peak C4 reflow temperature (namely, 350xc2x0 C.). The reaction products form the so-called flux residue, which by its chemical stability requires aggressive solvents for its removal. The presence of hydrogen greatly reduces this phenomenon by suppressing the transformation in abietic acid and, therefore, to implement the C4 joining process, IBM typically has used hydrogen environments.
2. Description of the Related Art
U.S. Pat. No. 5,334,260 (Stefanowski 1) and U.S. Pat. No. 5,281,281 (Stefanowski 2) disclose a no-clean, low residue, volatile organic compound free soldering flux and method of use.
Schneider, et al., discuss a rosin-free, low VOC, no-clean soldering flux and method or using the same in U.S. Pat. No. 5,571,340.
U.S. Pat. No. 5,417,771 (Arita, et al.) discloses a soldering flux.
A soldering flux composition is disclosed by Roberts in U.S. Pat. No. 4,360,392.
Mace, et al., discusses a thermally dissipated soldering flux in U.S. Pat. No. 5,004,508.
Takemoto, et al., discloses a soldering flux composition in U.S. Pat. No. 5,211,763.
U.S. Pat. No. 4,988,395 (Taguchi, et al.) discusses a water-soluble soldering flux and paste solder using the flux.
A tacky, no-clean thermally dissipated soldering flux is disclosed by Gutierrez, et a., in U.S. Pat. No. 5,129,962.
Minihara, et al., discloses a soldering flux composition and solder paste composition in U.S. Pat. No. 5,334,261.
A processing method for the manufacturing of electronic components using a soft soldering flux based on carboxylic acid is discussed in U.S. Pat. No. 5,116,432.
Schneider, of et al., discuss a no-clean soldering flux and method of using the same in U.S. Pat. No. 5,297,721.
U.S. Pat. No. 4,278,479 (Anderson, et al.) discloses an organic acid activated liquid solder flux.
One embodiment of the present invention is a fluxing composition comprising a high molecular weight carboxylic acid that forms a combination of carboxylate salts and unreacted acid anhydrides when applied to a solder alloy and exposed to temperatures in the range of about 150 to 350xc2x0 C. in an inert atmosphere and a carrier fluid comprising a mixture of organic solvents that is heat stable and non-reactive with the solder alloy and has a high viscosity at room temperature.
Another aspect of the present invention is a fluxing composition comprising (a) a mixture comprising a monocarboxylic acid having at least 8 carbon atoms, a hydroxycarboxylic acid and a dicarboxylic acid, the mixture forming a combination of carboxylate salts and unreacted acid anhydrides when applied to a lead-tin solder alloy and exposed to temperatures in the range of about 150 to 350xc2x0 C. in an inert atmosphere, and (b) a carrier fluid comprising a mixture of organic solvents comprising an alcohol, a dihydroxyalkane of from 2 to 6 carbon atoms, and a dihydroxypolyether of from 4 to 8 carbon atoms. The carrier fluid is heat stable and non-reactive with the solder alloy. The flux composition has a viscosity of about 6000 to about 8500 centipoise at room temperature.
Another aspect of the present invention is an integrated circuit assembly comprising an integrated circuit comprising a chip attached to a substrate by a plurality of solder joints and a thin layer of a residue that is reactive with an epoxy used in bonding the chip to the substrate.
Another aspect of the present invention is a method for preparing a bonded pair of surfaces. A thin film of a flux residue is prepared on at least a portion of one or both surfaces to be bonded. The residue is reactive with an epoxy used to bond the surfaces wherein the epoxy lies between the two surfaces. The epoxy is cured in the presence of the residue under conditions sufficient to cause the bonding of the epoxy to the surfaces.